| Direct borohydride fuel cell (DBFC) using alkali metal borohydride as fuel has attracted much attention in recent years. It has a high energy conversion efficiency and theoretical specific energy, fast electrode reaction rate and easy storage and transport of fuel. For these reasons, DBFC is a potential power supplier in portable applications. The anodic oxidation reactions of BH4- is involved with a novel and complex reaction system of three hydrogen valence state transformation among protide (H-)-protium (H0?)-proton (H+), which related to borohydride direct electrochemical oxidation and hydrolysis reaction with hydrogen evolution followed hydrogen electrochemical oxidation reaction. The anode catalytic materials are very different from traditional metal or metal oxide. The key for DBFC commercial application is to depress hydrogen evolution and reduce costs of anode catalysts. In this paper,we want to explore the possibility using low-cost Cu metal as the anode catalysts for DBFC, study the oxidation behavior and reaction kinetics of borohydride on Cu, and the performance of different metal-doped copper-based alloy electrode.In present dissertation,the electrochemical behavior of NaBH4 alkaline solution on copper was tested by CV, EIS and polarization curve. A multi-step mechanism for the oxidation of BH4- on Cu anode in NaOH solution was presented. In addition, polarization data of BH4- oxidation on Cu anode in alkaline solution were measured at steady state by a self-made experimental cell, and then the relation between hydrogen evolution rate and the anode current at different overpotential regions was analyzed in detail. It was found that the BH4- oxidation mechanism is a multi-step consecutive reaction including electrochemical and prepositive non-electrochemical elementary reaction steps. The result shows that the rate determining step will be the electro- oxidation of adsorbed sodium borohydride at low overpotential regions and it will change to sodium borohydride or hydroxyl activation adsorption at high overpotential. It is a transition area without rate determine step at middle overpotential. The apparent reaction order with respect to BH4- is approximate to zero at low overpotential and it will change to 1 at high overpotential, and it is 0-1 reaction order at middle overpotential. The limiting current density is caused by a limiting elemental step rather than by external mass transport. The apparent number of released electron n by the reaction was calculated by using the measured amounts of hydrogen and coulomb. Higher value of n was obtained at lower [BH4-]/ [OH-] ratio, higher temperature as well as higher over potential. Under the experimental conditions, the value of n varies from 0 to 7, while in the transitional region it is just in the range of 3 to 5. Based on the proposed reaction mechanism, the kinetic expression containing both coverage ratiosθB andθOH of BH4? ,ad and OHa?d adsorbed on the surface of Cu anode was deduced by the steady state approximation theory. Furthermore the kinetic parameters of elementary reaction steps were regressed from the experimental polarization data, and the theoretical polarization curves are calculated then compared with the experimental data. The theoretical calculation is in good agreement with the experimental results. At the same time, the reactions at Cu electrodes in alkaline borohydride solutions are successfully decomposed into two component reactions, i.e., borohydride oxidation and hydrogen electrode reaction by the current decomposition method. The relative contribution of these two reactions to the total current was determined, and the variation of the apparent number of electrons in borohydride oxidation was analyzed.On this basis,the electrochemical oxidation of BH4- in 2 M NaOH on carbon supported copper doped with Pt or Ag (catalytic and non-catalytic materials, respectively, for BH4- hydrolysis accompanied by H2 evolution) has been studied. It was found that that the performance of copper-based alloy electrodes can be improved by doping elements Ag or Pt. The DBFC with Cu-Pt/C as the anode catalyst showed the higher catalytic oxidation activity than that with Cu/C or Cu-Ag/C.
|
PDF Full Text Request